Instantaneous hot water: what it does well, and what it caps
The word promises hot water the moment you open the tap. What it actually means is that nothing is stored — which fixes one problem completely and introduces a different one that nobody mentions until the second shower goes weak.
An instantaneous hot water system heats water only while a tap is open, so it never runs out — but it delivers a capped number of litres per minute, and everything running at once has to share that cap. A storage system is the mirror image: it holds a set volume of already-hot water that several outlets can draw on hard at the same time, and when that volume is spent you wait for it to recover. Neither is better. The right one is decided by a single question about your household: do you need several outlets running at once, or a lot of hot water in one short burst?
What does "instantaneous" actually mean?
It means no stored hot water. Cold water enters the unit, passes across a burner or an element, and leaves hot — heating happens in the seconds it takes to travel through. The same systems are sold as continuous flow, and that is the more honest of the two names, because the water is continuous and the delivery is not instant.
This trips people up constantly: an instantaneous unit does not give you hot water at the tap any faster than a tank does. The wait at the tap is the cold water sitting in the pipe between the unit and the outlet, and that pipe run is the same either way. If your complaint is "it takes forever to get hot at the far bathroom", the system type is not the cause — the distance is.
Instantaneous or storage: how they differ in daily life
| Instantaneous (continuous flow) | Storage | |
|---|---|---|
| Can it run out? | No | Yes, then it recovers |
| The limit is… | Flow rate — litres per minute | Volume — how much is in the tank |
| Two showers at once | Each gets roughly half the rated flow | Fine, until the tank is spent |
| Long teenage shower | No problem at all | The classic way to empty a tank |
| Space needed | A wall box | A tank footprint |
| Standing loss | None — cold between uses | Yes — it keeps water hot around the clock |
| Needs electricity? | Yes, for ignition and controls | Not always, on gas storage |
Why flow rate is the number to insist on
A continuous flow unit is rated in litres per minute at a stated temperature rise — how many degrees it lifts the incoming water. Both halves of that matter, and the headline figure quietly assumes the friendly half:
- Winter mains water is colder, so lifting it to the same tap temperature takes more work and the same unit delivers fewer litres per minute in July than in January.
- Simultaneous outlets split the flow. A shower plus the kitchen tap share one cap. An undersized unit rarely runs properly cold — it feels weak and lukewarm, which is the most common complaint about this type of system and is almost always a sizing decision made at the quote stage.
So ask for the litres-per-minute figure at a realistic winter temperature rise for where you live, and size on how many hot outlets run in the same twenty minutes rather than on how many people live in the house. Two people on the same morning schedule can need more capacity than four who shower at different times.
Gas or electric instantaneous?
Both exist and they are not interchangeable. Gas continuous flow is the common whole-house version in Australia: a burner can deliver a lot of heat quickly, so one wall unit can serve a house. There is more on sizing, LPG and flue clearances in our guide to gas hot water systems.
Electric instantaneous works on the same principle but has to pull all that energy from your electrical supply for as long as the tap is open, with no tank to have stored it up in advance. That is why electric units of this type are normally fitted at a single outlet — a laundry trough, a remote kitchen sink, a granny flat — rather than serving a whole house. If someone proposes one whole-of-house, treat it as a question about your supply and switchboard capacity first, and get the answer in writing before the plumbing is quoted.
Three quirks of continuous flow worth knowing before you buy
- No power, no hot water. A continuous flow unit is an appliance with an electric brain — ignition and controls need a power point even on gas models. After a blackout or a tripped circuit the gas can be flowing perfectly and you will still have cold water. It is the first thing to check.
- There is a minimum flow to fire. The unit needs a certain flow through it before it starts heating at all, which is why a barely-open tap can run cold and why a slow trickle for shaving may never heat.
- The cold water sandwich. A slug of cold arriving mid-shower after a short pause happens because the unit shut down and restarted between draw-offs. It is characteristic behaviour of the type, not necessarily a fault.
Does instantaneous use less energy?
It removes one specific loss: standing loss, the heat a full tank sheds to the air around it all day whether anyone uses hot water or not. That is a real mechanism and it is the honest efficiency argument for the type. What it is not is a guarantee of a cheaper bill — that depends on your fuel, your tariff, how much hot water you actually use and, if you have rooftop solar, whether a tank could have been heated in the middle of the day for very little. A heat pump attacks the problem from the other direction entirely, by using roughly a third of the electricity for the same heat. We do not print running costs or rebate amounts here, because they vary by state and by year.
Which one should you choose?
- Instantaneous suits households whose hot water use is spread out, long single showers, small dwellings, tight outdoor spaces, and holiday houses that sit unused for weeks and would otherwise keep a tank hot for nobody.
- Storage suits households with a morning rush of simultaneous outlets, houses without gas where a heat pump makes more sense, and anyone with rooftop solar who wants to bank the heat while the sun is up.
How far can you take this yourself?
The whole decision. Count the outlets that run in the same twenty minutes, ask for litres per minute at a winter rise rather than the headline, check whether there is a power point at the unit, and get the flow rate, the siting clearances, the tempering valve and the compliance certificate written onto the quote before you accept it. None of that needs a licence, and it is where the outcome is actually decided.
This far, and no further. Installing or replacing a hot water system is licensed plumbing work in every Australian state and territory; a gas appliance additionally needs gasfitting authorisation and the electrical connection is licensed electrical work. Do not open a burner compartment, relight a pilot, or open the electrical enclosure on an electric unit. The rules and the paperwork are set by each state and territory — check with the plumbing regulator where you live, ask for the licence number, and keep the compliance certificate with the property records.
Frequently asked questions
What is the difference between instantaneous and continuous flow hot water?
Nothing — they are two names for the same type of system, which heats water only while a tap is open and stores none. Continuous flow is the more accurate name, because the water supply is continuous but the delivery at the tap is not instant: the wait is the cold water sitting in the pipe between the unit and the outlet.
Does instantaneous hot water ever run out?
No, and that is its main advantage — you can shower for as long as you like. What it does instead is cap the flow rate in litres per minute, so anything running at the same time shares that cap. Two showers at once means each gets roughly half, which feels weak and lukewarm rather than cold.
Is instantaneous hot water faster at the tap than a tank?
No. The delay between opening the tap and hot water arriving is the cold water already sitting in the pipe, and that pipe run is identical whichever system feeds it. If hot water takes a long time to reach a distant bathroom, the cause is the distance, not the type of system.
Why does my continuous flow hot water run cold for a moment mid-shower?
That is the cold water sandwich: the unit shut down when the tap was briefly closed or the flow dropped, and the slug of water that had already cooled in the pipe arrives before the unit fires again. It is characteristic of the type rather than a fault, although a unit doing it constantly is worth mentioning to a plumber.
Can I run a whole house on an electric instantaneous hot water unit?
Usually not. Heating water on demand with electricity requires a large sustained draw for as long as the tap is open, with no tank to have stored the energy in advance, so these units are normally fitted at a single outlet such as a laundry trough or a granny flat. Whole-of-house use is a question about your electrical supply and switchboard capacity, and needs to be confirmed in writing before the plumbing is quoted.
Sources
What this page explains is based on these official sources. If you check one and it does not match what you read here, send it to us and the article gets corrected.
- Australian Government, energy.gov.au. Hot water systems — types, installation and energy performance
- Australian Building Codes Board. National Construction Code — Volume Three (Plumbing Code of Australia)